1. Field of the Invention
The present invention relates to a process and device for displaying symbols by means of a liquid crystal matrix.
It applies more especially, but not exclusively, to the construction of sighting pieces or collimators mounted on vehicles, for example helicopters, in which it is necessary to superimpose visible symbols on the outside environment, for example on the external landscape, in the field of vision of an operator, for example the pilot.
2. Description of the Prior Art
It is generally known that numerous display devices or sighting pieces of this kind are already known using electro mechanical systems or cathode tube systems for generating the symbols.
In the electro mechanical systems, the symbols are formed by means of independent optical masks, either mobile or fixed. Usually, the movements of these symbols are provided by means of mobile assemblies sometimes equipped with mirrors. However, the number of symbols generated by these systems is limited because of the complexity and because of the number of mechanical assemblies required. Furthermore, these systems are relatively space consuming and suffer from a certain fragility.
In cathode tube systems, the screen of the cathode tube on which the symbols are generated is disposed in the object focal plane of an optical system serving for example for projecting these symbols on a semi-reflecting mirror. This solution has the advantage of displaying very complex systems of symbols with high definition. On the other hand, it proves to be costly and space consuming, which considerably limits its field of application. To try and overcome these drawbacks attempts have been made to construct display devices using, in optical systems similar to those used for the cathode tubes, liquid crystal matrices which are much less expensive and less space consuming than cathode ray tubes.
It will be recalled here in this connection that a liquid crystal matrix is formed from two transparent parallel glass plates which sandwich therebetween a liquid crystal. Each of these glass plates supports a network of addressable electrodes forming the lines and the columns of the matrix, which are connected to one or more electronic control circuits.
Furthermore, two light polarizers with parallel axes are disposed on each side of the glass plates. The effect used, well known under the name of "twisted nematic" is the following:
At rest, the liquid crystal is orientated so that the molecules rotate through 90.degree. from one edge to the other of the matrix.
Thus, a light beam applied perpendicularly to the matrix thus formed will first of all be polarized rectilinearly through the first polarizer. The polarized light will then be rotated through 90.degree. as it passes through the liquid crystal edge. The polarized light beam will then reach the second polarizer orientated perpendicularly to its axis of polarization. Thus, at the level of the second polarizer, extinction of the light beam is obtained. This extinction is maintained as long as the electric field between the electrodes forming the lines and the columns of the matrix is less than a predetermined electric field, generated by a threshold voltage, Vth.
By applying a sufficiently high electric voltage Von between a line electrode and a column electrode, the molecules will be aligned, at the level of the line and column intersection, along the electric field produced so that, in this zone, the light will not be deflected and will be orientated parallel to the axis of the second polarizer. The second polarizer will then let pass an elementary light dot or pixel (activation of a pixel).
The de-activation or extinction of the pixel is then obtained when the voltage Voff at the level of the pixel is less than a voltage Vth characteristic of the liquid crystal used.
At the present time, the only known way of generating an image on a liquid crystal matrix without memory effect, consists in effecting activation line by line, in which one line is selected at a time and all the electrodes of the columns are activated. Thus, for a matrix of n lines and m columns the multiplexing rate is n.
In addition, in the liquid crystal matrices used at the present time this multiplexing rate is limited by:
the frequency of refreshment of the image, which is related to the response time of the crystal, and PA1 the contrast which reduces when the multiplexing rate increases.
It has been worked out that, for this method of activation, the maximum contrast is obtained by the formula (Plesko's formula) ##EQU1## It has been proven in practice that the voltage Von is limited by the supply voltage of the "drivers". This is why, at the present time, the multiplexing rate is limited so that the number of lines is itself limited. Consequently, these liquid crystal matrices do not allow images to be obtained having sufficient definition for numerous applications, more especially for constructing sighting pieces of the type mentioned above.